1. Field of the Invention
The present invention relates to a photosensor-amplifier device that converts an optical signal incident thereon into an electric signal and that then amplifies the electric signal for output.
2. Description of the Prior Art
First, a conventional photosensor-amplifier device will be described with reference to FIGS. 4A and 4B. FIG. 4A is a schematic sectional view showing the structure of a principal portion of a conventional photosensor-amplifier device, and FIG. 4B is an equivalent circuit diagram of the photosensor-amplifier device shown in FIG. 4A. A common photosensor-amplifier device as shown in these figures is composed of a photodiode chip 100 functioning as a photoelectric conversion element and an IC chip 200 incorporating an amplifier circuit and other components, with the photodiode chip 100 and the IC chip 200 sealed in a single package.
The photodiode chip 100 has an N-type semiconductor substrate 101 and a P-type semiconductor region 102 formed in a top portion of the substrate 101, the PN junction in between constituting a photodiode PD. The top surface of the photodiode chip 100 is coated with an insulating film 103, of which a small portion above the P-type semiconductor region 102 is removed. In this portion where the P-type semiconductor region 102 is exposed, the anode electrode 104 of the photodiode PD is provided. On the other hand, the bottom surface of the substrate 101 is die-bonded to a frame 50, and a supply voltage VDD is applied to the frame 50 from outside. That is, the frame 50 serves as the cathode electrode of the photodiode PD.
The anode electrode 104 of the photodiode PD is electrically connected by way of a wire W to an electrode 201 of the IC chip 200. As shown in FIG. 4B, the IC chip 200 incorporates an amplifier circuit AMP and a resistor R, and the electrode 201 is connected to the input terminal of the amplifier circuit AMP and also through the resistor R to ground.
In this photosensor-amplifier device built as described above, an optical signal incident on the photodiode chip 100 is sensed by the photodiode PD and is detected as a current signal that flows through the photodiode PD. The current signal thus obtained as a result of photoelectric conversion performed in the photodiode chip 100 is then fed by way of the wire W to the IC chip 200, where the current signal is converted into a voltage signal by the resistor R. This voltage signal is then amplified to a predetermined voltage level by the amplifier circuit AMP, and is then fed to a signal processing circuit (not shown) provided in the succeeding stage.
In this conventional photosensor-amplifier device built as described above, the path connecting the photodiode chip 100 to the IC chip 200 (i.e., the wire W and other wiring elements) has a high impedance, and therefore electromagnetic noise coming from outside the device or electromagnetic noise generated inside the device tends to cause electromagnetic induction whereby noise signals tend to be induced in the wire W and other components. Moreover, the path connecting the photodiode chip 100 to the IC chip 200 is susceptible also to noise signals induced by the coupling capacitance that accompanies the path.
Despite these facts, the conventional photosensor-amplifier device is provided with no means of reducing such noise signals, and therefore noise signals are amplified, unchecked, by the amplifier circuit AMP and tend to cause malfunctioning of the IC chip 200. To solve this problem, some measure against electromagnetic noise, such as an electromagnetic shield, is essential, which inconveniently increases the total number of components, and thus the cost, of the photosensor-amplifier device.
Moreover, as shown in FIG. 4A, in the photosensor-amplifier device built as described above, the anode electrode 104 of the photodiode PD is connected to the electrode 201 of the IC chip 200 by way of a single wire W. Thus, the wire W is, at both ends, die-bonded directly to the anode electrode 104 and to the electrode 201, respectively.
In the wire-bonding process of this wire W, first, one end of the wire W is bonded to one of the anode electrode 104 and the electrode 201 (this operation is called the first bonding), and then the other end of the wire W is bonded to the other of those electrodes (this operation is called the second bonding). Here, on the chip where the wire W was bonded as the second bonding, it is subsequently necessary to cut the wire W. Inconveniently, the mechanical force accompanying the wire cutting here is applied to the chip and may cause chip breakage.
An object of the present invention is to provide a photosensor-amplifier device that, despite having a photoelectric conversion circuit and an amplifier circuit connected together by way of a wire, is less likely than ever to malfunction under the influence of noise signals induced in the wire and other components.
Another object of the present invention is to provide a photosensor-amplifier device that is less likely than ever to suffer chip breakage in a wire-bonding process.
To achieve the above object, according to the present invention, a photosensor-amplifier device has a photoelectric conversion circuit that converts an optical signal into an electric signal, a first electrode by way of which the electric signal is extracted from the photoelectric conversion circuit, a second electrode that is not directly connected to the electric signal, an amplifier circuit that has a first input terminal and a second input terminal and that amplifies and then outputs the difference between the electric signals fed to the first and second input terminals, a first wire that connects the first electrode to the first input terminal, and a second wire that connects the second electrode to the second input terminal.